Oxy-acetylene torch tip manufacturers publish charts that tell us the ideal applications for each tip size, which is quite useful on its own. After all, a small tip may be great for a ⅜” liquid line but would hardly make a dent in 2” commercial refrigeration piping. But we can also take the information from those charts a step further to determine how many BTUs of heat the tip can emit. It can put our torch work into perspective and help us be mindful of metal melting points, nearby components that may need protection, etc.  

First, let’s dive into why understanding heat transfer is especially important for brazing copper.

Basic Properties of Copper

Copper has two features that we really need to watch out for when we’re brazing: its thermal conductivity and its melting point.

Thermal conductivity is a material’s ability to transfer heat. Copper is extremely conductive—about 4x more conductive than brass. Heat moves from the torch to the copper quickly and easily.

The melting point is the point at which a solid becomes a liquid. Copper has a relatively high melting point compared to other base metals, like aluminum. Its melting point is 1984°F. Compare that to brass, which is around 1700°F, and aluminum, which tops out at a mere 1220°F. (You can find some extra information about these metals and torch work in our 3D-animated video about oxy-acetylene brazing.) The high-temperature brazing alloys we’re working with typically melt between 1200 and 1500°F, so we want to get the copper hot enough to melt those and draw them into the joint but not melt the copper itself. 

However, the average temperature of the flame from an oxyacetylene torch is over 5000°F. Since copper has very high thermal conductivity and a melting point of 1984°F, that’s a bit of a problem if we’re trying to avoid burning through the copper. 

In order to apply the heat in a way that DOESN’T overheat the copper, we need to use the appropriate torch tip for the tubing size. The tip design affects how that heat is applied by restricting the flow rate.

Tip Sizes

Your torch tips will have numbers on them. These numbers correspond to different sizes, which are suitable for different copper tubing diameters. Different tip sizes don’t change the temperature of the flame, but they control how much of that heat comes out of the tip and makes it to the pipe. Smaller tips have a narrower opening to limit where that heat is applied (and lower pressures than the REALLY BIG tips).

Unsurprisingly, smaller tips are better for smaller pipe diameters in residential systems, and larger tips are better for larger pipes in heavy commercial HVAC/R systems.

As you can see, a size 0 tip in the chart above is great for installing residential systems; it’s designed for brazing jobs on ¼” to 1” tubing, which is right in residential’s wheelhouse. But it won’t make much headway in commercial refrigeration with 2” pipes. A size 4 tip is much better for brazing the large pipes we need in commercial refrigeration, but it would take very little for it to burn right through the copper we use for residential line sets. 

The tip size will also dictate your regulator settings, such as in the manufacturer chart below (fuel & oxygen PSIG):

As you can see, larger tip sizes consume more fuel (acetylene consumption SCFH). Greater fuel consumption means a higher BTU output.

BTU Outputs

You can calculate the approximate BTU output by multiplying the acetylene SCFH by 1470, which is the gross BTU value of 1 SCFH of acetylene. Oxygen does NOT add BTUs to the equation; it’s absolutely needed for combustion, but the acetylene SCFH is the one that directly correlates to the BTU output. 

Based on Uniweld’s chart for Type17 tips above, a size 0 tip that consumes a maximum of 4 SCFH of acetylene only has an approximate BTU output of 5,880. 

However, a size 4 tip that could consume anywhere from 10–25 SCFH will have a MUCH higher BTU output. If that size 4 tip consumes 18 SCFH of acetylene, that BTU output would be over 26,000. 

When selecting your torch tip, remember that the heat is going from the tip to the base metal—and whatever may be in the immediate area, like brass valves. A tip size that’s too large can also cause some serious damage to nearby components if you’re not careful. 

Does knowing the exact BTU output have a super practical use? Not really for the work we do. But it helps put torch tip size into perspective and why it’s important not to pick a tip that’s too big and will do a number on the tubing and nearby valves or compressor paint. Plus, it’s pretty neat that a 4 SCFH BTU/hr output is almost the equivalent of a half-ton of cooling capacity.